1. Field of the Invention
The present invention relates to a retardation film used in a state installed in a display device such as a liquid crystal display and a method for producing the same, an optical functional film, a polarizing film and a display device.
2. Description of the Related Art
As a conventional common liquid crystal display, as shown in FIG. 22, one comprising a polarizing plate 102A on an incident side, a polarizing plate 102B on an outgoing side and a liquid crystal cell 104 can be presented. The polarizing plates 102A and 102B are constructed so as to selectively transmit only the linear polarization having an oscillation surface in a predetermined oscillation direction (shown schematically by an arrow in the figure), and are disposed facing with each other in a cross nicol state such that the oscillation directions thereof have a relationship perpendicular with each other. Moreover, the liquid crystal cell 104, including a large number of cells corresponding to pixels, is disposed in between the polarizing plate 102A and polarizing plate 102B.
Here, in such a liquid crystal display 100, for example, in the case of the liquid crystal cell 104 adopting a VA (vertical alignment) system, in which a nematic liquid crystal having a negative dielectric anisotropy is sealed (the liquid crystal director is shown schematically by a dotted line in the figure), the linear polarization transmitted through the polarizing plate 102A on the incident side is transmitted without being its phase shifted, at the time of being transmitted through a non-driven state cell part among the liquid crystal cell 104, so as to be blocked by the polarizing plate 102B on the outgoing side. In contrast, at the time of being transmitted through a driven state cell part among the liquid crystal cell 104, the phase of the linear polarization is shifted so that light of an amount according to the phase shift amount is transmitted through and outgoes from the polarizing plate 102B on the incident side. Accordingly, by optionally controlling the driving voltage of the liquid crystal cell 104 per each cell, a desired image can be displayed on the side of the polarizing plate 102B on the outgoing side. The liquid crystal display 100 is not limited to ones having the above-mentioned configuration of the light transmission and blockage. On the other hand, a liquid crystal display, in which the outgoing light from the non-driven state cell part among the liquid crystal cell 104 is transmitted through and outgoes from the polarizing plate 102B on the outgoing side, and in which the outgoing light from the driven state cell part is blocked by the polarizing plate 102B on the outgoing side, is also proposed.
Considering the case of the linear polarization transmitted through the non-driven state cell part among the liquid crystal cell 104 of the above-mentioned VA system, since the liquid crystal cell 104 have the property of double refraction so that a refractive index in the thickness direction and a refractive index in the plane direction differ with each other, incident light along the normal line of the liquid crystal cell 104, among the linear polarization transmitted through the polarizing plate 102A on the incident side, is transmitted without the phase being shifted. However, the phase of the incident light entering in an inclined direction to the normal line of the liquid crystal cell 104, among the linear polarization transmitted through the polarizing plate 102A on the incident side, is shifted when the light is transmitted through the liquid crystal cell 104 so as to be elliptically polarized. This phenomenon is derived from the liquid crystal molecules, aligned in the perpendicular direction in the liquid crystal cell 104, acting as a positive C plate. The magnitude of the retardation generated with respect to the light transmitted through the liquid crystal cell 104 (transmitted light) depends also on the double refractive value of the liquid crystal molecules sealed in the liquid crystal cell 104, the thickness of the liquid crystal cell 104, the wavelength of the transmitted light or the like.
Due to the above-mentioned phenomenon, even when a cell of the liquid crystal cell 104 is in the non-driven state so that the linear polarization is inherently transmitted as it is, so as to be blocked by the polarizing plate 102B on the outgoing side, a part of the light, outgoing in the inclined direction to the normal line of the liquid crystal cell 104, is leaked form the polarizing plate 102B on the outgoing side.
Therefore, in the above-mentioned conventional liquid crystal display 100, there is a problem of a display quality deterioration of an image observed from the inclined direction to the normal line of the liquid crystal cell 104 (a problem of the visual angle dependency), compared with an image observed from the front side, mainly due to the contrast decline.
To improve the problem of the visual angle dependency in the above-mentioned conventional liquid crystal display 100, various techniques have been developed so far. As an example, as disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 3-67219 and 4-322223, a liquid crystal display, using a retardation layer (retardation layer showing the property of double refraction) having a cholesteric regularity molecular structure, is known. By disposing such the retardation layer in between the liquid crystal cells and the polarizing plates, an optical compensation is carried out.
Here, in the retardation optical element having the cholesteric regularity molecular structure, the selective reflection wavelength, represented by λ=nav·p (p: helical pitch in a helical structure of the liquid crystal molecule, nav: average refractive index in an orthogonal plane to the helical axis), is adjusted so as to be shorter or longer than the wavelength of the transmitted light, for example as disclosed in JP-A Nos. 3-67219 or 4-322223.
In contrast, for example as disclosed in JP-A No. 10-312166, a liquid crystal display, in which the optical compensation is carried out by using a retardation layer (retardation layer showing the property of double refraction) comprising a disc like compound and by disposing such retardation layer in between liquid crystal cells and a polarizing plate, is also known.
In the above-mentioned retardation optical element, as in the case of the above-mentioned liquid crystal cells, the phase of the linear polarization incident, entering in an inclined direction to the normal line of the retardation layer, is shifted when it is transmitted through the retardation layer so as to be elliptically polarized. This phenomenon is derived from the molecular alignment of the cholesteric regularity and the disc like compound itself acting as a negative C plate. The magnitude of the retardation generated with respect to the light transmitted through the retardation layer (transmitted light) depends also on the double refractive value of the liquid crystal molecules in the retardation layer, the thickness of the retardation layer, the wavelength of the transmitted light or the like.
Therefore, by using the above-mentioned retardation layer, the problem of the visual angle dependency of the liquid crystal display can dramatically be improved by optionally designing the retardation layer such that the retardation generated in the VA system liquid crystal cells, which act as the positive C plate, and the retardation generated in the retardation layer, which act as the negative C plate, offset with each other.
In this case, the visual angle dependency of the polarizing plate can be improved, with the remaining positive plate C component and an A plate prepared separately, by making the sum of the retardation values in the thickness direction of the above-mentioned positive C plate and the above-mentioned negative C plate positive. That is, by making the absolute value of the retardation value in the thickness direction of the above-mentioned negative C plate smaller than the absolute value of the retardation value in the thickness direction of the above-mentioned positive C plate. The improvement of the visual angle dependency of the polarizing plate with the positive C plate and A plate is disclosed in, for example, J. Chen et al., SID98 Digest, p315 (1998) and T. Ishinabe et al., SID00 Digest, p1094 (2000).
However, in the above-mentioned retardation layer, there is a problem of an adhesion between the retardation layer and the base material (for example, the TAC (cellulose triacetate film) as the protecting film for the polarizing layer).
In order to solve the problem, as disclosed in for example JP-A No. 2003-207644, improvement of the adhesion, by treating the liquid crystal and the alignment film with heat, is proposed. However, in this method, when the base material is not a glass substrate but a base material having low moisture and heat resistance (for example, TAC), the base material is stretched or shrunk by the influence of the moisture so that the liquid crystal layer may be peeled off due to the above. And thus, it is not a satisfying method for base materials easily influenced by the moisture.
As a method free of the above-mentioned problems of the adhesion, for example as disclosed in JP-A Nos. 2000-111914 and 2001-249223, a method of forming a cellulose acetate film by mixing a retardation increasing agent in a cellulose acetate solution, at the time of producing a cellulose acetate film, can be adopted. However, by such method, since the retardation increasing agent should be mixed at the time of forming the cellulose acetate film, the amount of one lot is inevitably made larger. Therefore, there is a problem that it is difficult to easily obtain optional retardation for a small amount. Moreover, since the retardation increasing agent in general is hydrophobic, by mixing the same in the entirety, the front and rear surfaces of the cellulose acetate film become hydrophobic so that a problem is involved in that the adhering property at the time of laminating the retardation layer on a polarizing plate comprising a hydrophilic resin such as a polyvinyl alcohol. Furthermore, since the mixable amount of the retardation increasing agent is substantially limited, and thereby the retardation value to be obtained is limited as a result.
In contrast, use of a biaxial plate to function as a C plate and an A plate instead of using a combination of a C plate and an A plate as the compensation layer is disclosed in for example j. Chen et al., SID98 Digest, p315 (1998), T. Ishinabe et al., SID00 Digest, p1094 (2000) and Japanese Patent Application Laid-Open No. 2000-131693. However, according to the disclosed products, a problem of the limitation of the values of the thickness direction retardation and the in-plane direction retardation to be obtained and a problem of the difficulty in desirably controlling the values of the thickness direction retardation and the in-plane direction retardation independently are involved.